Switched capacitor transresistance amplifier

ABSTRACT

A transresistance amplifier particularly adapted for use in a radiation detection system. The amplifier includes a feedback gain stage with a switched capacitor load. The amplifier is arranged to provide an average detector voltage approximating zero thus substantially reducing detector noise and also providing a low equivalent input impedance for increasing injection efficiency. A switched capacitor output load is also provided which allows the total transresistance to be determined by simply selecting an appropriate capacitance value.

This is a division of application Ser. No. 505,633, filed June 20, 1983,now U.S. Pat. No. 4,567,363, issued Jan. 28, 1986.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to electronic amplifiers and, in particular, to atransresistance amplifier having a gain stage in its feedback path and aswitched capacitor load, the tranresistance amplifier being particularlyadapted for use in a radiation detection system.

2. Description of the Prior Art

The continuing goal of microelectronic designers is to increase thenumber of microelectronic circuits formed on a semiconductor chip ofdecreasing size, the circuits and their interconnections beingmanufactured simultaneously. A concurrent goal is to reduce the powerdissipation of the chip.

In many types of signal processing systems, such as monolithic infraredfocal plane signal processing systems used in space detectionapplications, for example, it is required that each component in thesystem be designed to optimize overall performance while reducing systempower dissipation. In the above mentioned system, the signal processingchip is intended to process the output of a plurality of independentphotovoltaic infrared detectors. The output of each detector is anelectric current which must be converted into a voltage by atransresistance amplifier connected thereto, the voltage output of theamplifier being applied to a high pass filter to remove signalinformation from stationary background sources. The output of the filteris coupled to external circuitry via a multiplexer and buffer. Thedesign of the transresistance amplifier in this system is thus based inpart on the characteristics of the detector and filter. Regarding thephotovoltaic detector, detector current is related to the incident lightflux, the voltage bias across the detector and, to a lesser extent, thereverse saturation current.

In order to reduce detector noise, it is necessary to operate thedetector with low (ideally zero) bias voltage. Operating the detectorinto a short circuit load would satisfy this requirement, the standardsolution to such a problem being to use an operational amplifier as theload. However, this is difficult to achieve in practice because theavailable chip area in this application is limited and is unable tosupport an operational amplifier. Further, even if an operationalamplifier could be utilized, it dissipates relatively large amounts ofpower. Therefore, it is desired to find a solution which permits thedetector to be operated at a relatively low voltage level which remainsessentially constant for a large variation in light flux while at thesame time minimizing power dissipation and required chip area.

Other constraints placed on the design of the transresistance amplifierin addition to those due to the characteristics of the filter anddetector include temperature (infrared detection applications requirethat the amplifier be in an extremely cool environment, (e.g., 77° K.),dynamic range, actual transresistance, injection efficiency, and thetype of active device (i.e., pMOS or nMOS) which can be used.Transresistance is a measure of the amount of AC output voltagegenerated for a given amount of AC input current, while injectionefficiency is a measure of the amount of AC current provided to thedetector load circuitry for a given amount of AC detector current.

A number of solutions have been proposed to provide a tranresistanceamplifier which meets the above constraints. One such solution utilizesa gain stage in the gate electrode of a MOSFET transresistanceamplifier, a second MOSFET with a resistive load being used as the gainstage. Although noise is reduced, the power dissipation due to theresistance load is relatively high, especially when considered in lightof the relatively small improvement in injection efficiency provided bythis arrangement. Another solution which is a variation on the abovesolution, utilizes a depletion mode MOSFET in place of the resistiveload of the above arrangement, thus providing a high equivalent loadresistance. However, power dissipation due to the resistance load isstill higher than desired.

An article "Mosaic Focal Plane Methodologies" by Wong et al.,Proceedings of the Society of Photo-Optical Instrumentation Engineers,Vol. 244, pp. 113-125 (1980) describes the use of Z-technology infabricating mosaic sensor focal planes, Z-technology allowing increasedsignal processing integrated circuit area per detector channel. Thearticle discloses a common gate reset transimpedance amplifier whichband limits the noise of the detector and preamplifier and includes asynchronously clocked switched capacitor filter to achieve the desiredfunction. As described in the article "MOS Switched-Capacitor Filters",Broderson et al., Proceedings of the IEEE, Vol. 67, No. 1, pp. 61-75(January, 1979), the switched capacitor essentially functions as aresistor, the resistance value of which is related to the value of theswitched capacitor and the switching frequency. Although thisconfiguration performs satisfactorily, the detector voltage is notaccurately controlled such that it is consistently maintained close tozero.

U.S. Pat. No. 4,100,407 to Takahashi discloses a circuit having acapacitor for comparing the output voltage of an operational amplifier,connected to a photosensor, with a reference voltage, and dischargingmeans operable under the control of the capacitor to discharge thecharge stored in a parasitic capacitor of the photosensor when thecircuit is energized for operation; U.S. Pat. No. 4,320,347 to Haguediscloses a switched capacitor employing an operational amplifier; andU.S. Pat. No. 3,988,689 to Ochi et al., U.S. Pat. No. 4,068,182 toDingwall et al. and U.S. Pat. No. 4,255,715 to Cooperman disclose theuse of switching capacitors. It should be noted that none of the patentreferences set forth hereinabove are concerned with providing atransresistance type amplifier which meet the design criteria set forthhereinabove.

SUMMARY OF THE INVENTION

The present invention provides an improved transresistance amplifier foruse in low power circuit applications. The transresistance amplifier("Tr amplifier") includes a gain stage in its feedback path whichenables the input impedance of the Tr amplifier to be reduced, the gainstage including a switched capacitor load. An integrating capacitor iscoupled across the output of the Tr amplifier. The gain stage and the Tramplifier are each connected to a switch, the on-off conditions of whichare under the control of an external switching signal. Input current tothe Tr amplifier from a current source is integrated by the dischargingof the integrating capacitor. At the end of the integration period thevoltage across the capacitor is sampled which represents the amount ofcurrent input to the amplifier. The input current initially tends tomake the input voltage rise, the voltage being amplified by the gainstage and being provided with an appropriate polarity to reduce theinitial input voltage, thus lowering the current flowing through the Tramplifier into the current source. This process continues in aregenerative manner until the input voltage is lowered to a smallaverage value, thus lowering the input impedance of the Tr amplifier.Since the capacitor across the feedback amplifier is continuously beingswitched between two voltage levels, the input voltage does not reach asteady state value but varies throughout the switching cycle. Theaverage input voltage is reduced, however, yielding the desired lowinput impedance load, the total transresistance of the Tr amplifierbeing related to the value of the integrating capacitor and the systemswitching frequency.

The present invention thus provides an effective technique forcontrolling the equivalent input impedance of a transresistanceamplifier using switched capacitors and a gain stage in the feedbackpath of the transistor amplifier. In the specific adaptation wherein thetransresistance amplifier is coupled to a photovoltaic detector, theinput impedance (and voltage across the detector) is maintained at a lowvalue thus substantially reducing the noise characteristics of thedetector and increasing the system current injection efficiency.

The implementation of the above by utilizing a switched capacitorprovides additional advantage. In particular, large values of resistancecan be simulated using a component (capacitor) which can be easilyimplemented in monolithic form and which provides minimal powerdissipation. Further, unlike discrete resistors, the simulatedresistance is not affected by temperature variations. The use of aswitched capacitor load in the feedback path also provides higher gainthan that provided by the use of a MOSFET dynamic load.

The transresistance amplifier of the present invention is particularlyuseful in very low power (monolithic) applications employing MOSFETtransistors operating in the sub-threshold region, and also operateswell at cryogenic temperatures, making it an ideal monolithicphotovoltaic detector amplifier.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the invention as well as other objects andfurther features thereof, reference is made to the following descriptionwhich is to be read in conjunction with the accompanying drawingwherein:

FIG. 1 is a prior art direct injection amplifier (transimpedanceamplifier);

FIG. 2 is an improvement over the device shown in FIG. 1 and utilizes again stage in the gate circuit;

FIG. 3 illustrates the theory of switched capacitor operation;

FIG. 4 is a schematic diagram of a transresistance amplifier inaccordance with the teachings of the present invention;

FIG. 5 is an equivalent circuit of a photovoltaic detector utilized withthe transresistance amplifier of the present invention;

FIG. 6 is a graphical representation of the threshold and sub-thresholdoperating characteristics of a typical MOSFET; and

FIGS. 7(a)-7(e) are graphical representations of waveforms associatedwith the amplifier shown in FIG. 4.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, a prior art transresistance amplifier 8,referred to as either a direct injection or transimpedance amplifier, isillustrated. Amplifier 8 is a circuit based upon an enhancement moden-MOS transistor 10 having a bias voltage, V_(bias), applied to the gateelectrode. It should be noted that p-MOS transistors with appropriatebiasing could also be utilized. In both cases, the term MOSFET isgeneric. The drain electrode of transistor 10 is connected to node 16 towhich are connected one terminal of capacitor 12 and the sourceelectrode of enhancement mode n-MOS switching transistor 14. The otherterminal of capacitor 12 is grounded. The drain electrode of a thirdenhancement mode n-MOS switching transistor 18 is also connected to node16. The source electrode of switching transistor 18 is connected to oneterminal of capacitor 20, the other capacitor terminal being grounded. Aswitching pulse signal .0.₁ is applied to the gate electrode oftransistor 18 and a switching pulse signal .0.₂, having a pulse widthtypically of 100 μs, is applied to the gate electrode of transistor 14.

In the embodiment illustrated, amplifier 8 is connected to aphotovoltaic detector 22. Typically, detector 22 is responsive toinfrared radiation and may, for example, be incorporated in a signalprocessing chip which processes the outputs of, for example, thirty-twoindependent photovoltaic detectors. The current output of each detectoris converted to a voltage by a plurality of amplifiers like amplifier 8,the outputs of which can be applied to a corresponding number of highpass filters to remove the stationary background information. The outputof each filter, in turn, is connected to a mulitplexer and buffer, theoutput of which corresponds to the output of a selected detector.

As set forth hereinabove, photovoltaic detectors perform best in mostinfrared detection systems when operated in the current mode. In thismode, the detector is optimally biased near zero volts and the detectorsignal current is coupled and converted to a useful voltage by amplifier8. As explained hereinafter with reference to FIG. 3, a switchedcapacitor arrangement comprising capacitor 12, transistor 14, resetsupply voltage V_(rst) and switching signal .0.₂, operate in a mannersuch that an effective resistance, having a value equal to 1/C₁₂f.sub..0.2, wherein C₁₂ is the capacitance value of capacitor 12 andf.sub..0.2 is the switching frequency, loads transistor 10. Thecomponents used in this equivalent resistive load implementation can befabricated in monolithic form on a small chip area with reduced powerdissipation.

Switching signal .0.₁ enables the voltage stored on capacitor 12 to besampled and stored in capacitor 20 as V_(f) for eventual readout bycircuitry external to the signal processing chip. After sampling, signal.0.₁ causes transistor 18 to be switched off and switching signal .0.₂,a predetermined time period thereafter, switches transistor 14 on,effectively resetting capacitor 12 for the next integration period.

FIG. 2 improves upon the amplifier 8 shown in FIG. 1 by providing a gainstage in the gate circuit of transistor 10. It should be noted thatprimed reference numerals refer to corresponding components in each ofthe figures. In particular, a MOSFET transistor 24 transistor isutilized as the gain stage and is coupled between the gate electrode oftransistor 10 and detector 22, as illustrated. A resistance 26 isincluded in the load circuit of transistor 24. Although thetransresistance amplifier 8' shown in FIG. 2 has a reduced inputimpedance as compared with amplifier 8 (FIG. 1) and thus has improvednoise characteristics, MOSFET transistor 24 is typically operated in theabove threshold region to achieve a gain (usually low) resulting in arelatively large power dissipation. Further, to increase the gain,larger chip areas than desired are required.

FIG. 3 explains, in simplified form, the concept of a switched capacitorresistor. A capacitor C has one terminal grounded and the other terminalconnected to a switch arm 30 which is switched between nodes A and B ata frequency f_(s). A source of voltage V₁ is connected to node A and asource of voltage V₂ is connected to node B. An average current I flowsto capacitor C, the magnitude thereof given by ##EQU1## wherein q is thecharge collected on the capacitor plates. Equation (2) can be rewrittenas:

    I.sub.av R.sub.eq =V.sub.1 -V.sub.2                        (3)

where

    R.sub.eq =1/Cf.sub.s                                       (4)

wherein R_(eq) is the equivalent resistance value. It should be notedthat switched capacitor resistors behave as resistors only until thesignal frequency approaches one-half of the switching frequency f_(s).Thus, for a typical switching frequency of 5.0 Hz signal frequencies ofup to 2.5 Hz are of interest.

Referring now to FIG. 4, the transresistance amplifier 8" of the presentinvention is shown. The drain electrode of a MOSFET transistor 10" isconnected to the source electrode of a MOSFET switching transistor 14"and the source electrode thereof is coupled to node 32. Both the cathodeterminal of detector 22" and the gate electrode of a MOSFET transistor24" are connected to node 32. The source electrode of transistor 24" isconnected to bias voltage source V_(bias) and the drain electrodethereof is connected both to the source electrode of a second MOSFETswitching transistor 36 and to one terminal of capacitor 38, the otherterminal of which is at ground. A timing signal .0.₂ (t) suppliedtypically by a source external to amplifier 8" and having a repetitionfrequency f_(s) (FIG. 7(a)), is connected to the gate electrode ofMOSFET transistors 14 and 36. It should be noted that FET devices, otherthan MOSFET transistors could be utilized as the amplifiers and/orswitches. The remaining components of the circuit are connected asshown, in a manner similar to the corresponding components in FIGS. 1and 2.

Prior to going into the description of the circuit operation, a reviewof the characteristics of a typical photovoltaic detector and thecurrent characteristics of a typical MOSFET transistor would beinstructive. FIG. 5 represents an ideal model for detector 22". At agiven bias, the detector current I_(det) is proportional to the incidentlight flux. To reduce noise it is desirable to operate the detector intoa short circuit load (voltage across the detector, V_(det) =0). This isdifficult to achieve in practice because of chip area and powerconsiderations. Operational amplifiers have very high input impedanceand require a large chip area whereas bipolar transistors operate poorlyunder low temperature/low current conditions.

Thus, it would be desirable to have a circuit design that operates thedetector at a relatively low V_(det) that remains essentially constantfor a wide change in light flux, has relatively low power dissipationand does not require a large chip area. These advantageouscharacteristics are provided by the present invention. In essence, thecurrent I_(det) (or I_(ds)) into the detector is maintained at a lowaverage value over the switching cycle such that I_(det) closelyapproximates I_(ph), thus reducing the voltage across R_(r) (and thusV_(det)) towards zero.

FIG. 6 is a graph which shows a typical MOSFET transistor currentcharacteristic and illustrates how current varies in response to V_(gs)(the voltage between the gate and source electrode), the current I_(ds)being plotted as a logarithm function. Above the threshold voltageV_(t), typically 2.0 Vdc, current varies relatively slowly withincreased values of V_(gs). Below the threshold voltage (thesub-threshold range is highly non-linear) current decreases very rapidlyfor small changes in voltage. For example, if the transistor isoperating at V_(t) (here, 2.0 Vdc), I_(ds) typically has a value in therange of 100_(na). If V_(g) is slightly decreased to 1.84 Vdc forexample, I_(ds) typically falls to a value of 1 pa. Thus, for a slightchange in voltage, the current changes rapidly.

Returning to the operation of the circuit shown in FIG. 4, referenceshould be now be made addition to FIGS. 7(a) through (e). FIG. 7(a) is aplot of .0.₁ (t) with time, while FIG. 7(b) is a plot of .0.₂ (t) withtime, FIG. 7(c) is a plot of I_(ds) (t) with time, FIG. 7(d) is a plotof V_(det) with time, and FIG. 7(e) is a plot of V_(out) (t) with time.The time axes of FIGS. 7(a) through (e) align vertically to aid inillustration.

MOSFET transistors 14" and 36 function as switches under the control of.0.₂ (t). Initially, .0.₂ (t) causes both transistors 14" and 36 to beon for a short period τ (FIG. 7(a)) charging capacitor 12" to V_(rst)(FIG. 7(d)), typically 10 Vdc, and charging capacitor 38 to V_(dd),typically 3 Vdc.

At time τ, MOSFETS 14" and 36 are switched off. The 3 Vdc appearing atthe gate of MOSFET transistor 10" is above the threshold value of 2 Vdc(it is assumed that MOSFET transistors have a 2 Vdc threshold value),causing a large current, I_(ds) (FIG. 7(b)), to flow through transistor10". The current flowing through detector 22" initially tends to makethe detector voltage, V_(det) (FIG. 7(c)), rise and effectively mask thephotovoltaic current I_(ph). The detector current is integrated bydischarging capacitor 12" through transistor 10". The rise in detectorvoltage V_(det) (to approximately 1 Vdc) appears at the gate oftransistor 24". The resulting increased gate to source voltage oftransistor 24", typically 3 Vdc (assuming a V_(bias) of -2 Vdc), causesa large current to flow through transistor 24" from capacitor 38. Thisin turn decreases V_(g) from an initial value of 3 Vdc, which decreasesI_(ds) (FIG. 7(b)), further reducing V_(det) towards zero (FIG. 7(c)).The decreased value of V_(det) in turn causes a decreased current flowin transistor 24", further reducing the voltage across capacitor 38. Theprocess continues due to the negative feedback connection of MOSFETtransistor 24" with the current in transistor 24" being reduced to a lowvalue and the voltage across capacitor 38 stabilizing at some value(˜1.90 volts) near the threshold of transistor 10". I_(ds) in turnstabilizes at a value close to I_(ph) (I_(ph) is essentially constantdue to the constant radiation), V_(det) thus being reduced to an averagevalue close to zero (i.e., within a few millivolts as shown in FIG.7(c), such that detector noise is minimized. It should be noted thatwith V_(det) close to or equal to zero, I_(ph) is substantially the onlycurrent flowing into the source of transistor 10", thus providing anaccurate measure of the detected radiation. The entire process describedoccurs very rapidly.

At the end of the integration period (T_(int)), which is sufficient toallow I_(ds) to reach a value approximating I_(ph), the output voltagelevel V_(out) (FIG. 7(d)) across capacitor 12" is sampled by .0.₁ (t)switching on transistor 18" (FIG. 7(a). Capacitor 20" thereby providesan output V_(f) representing the detected radiation. Capacitor 12" isthen reset to V_(rst). It should be noted that V_(g) (and V_(det)) doesnot reach a steady state value and varies throughout the switchingcycle. However, the average detector voltage is reduced which yields thedesired low input impedance Z_(in).

As set forth hereinabove, the voltage on the drain of transistor 24",after capacitor 38 has been charged to V_(dd), rapidly decays towardV_(bias). However, this in turn causes the voltage across detector 22"to drop and consequently the drain current of transistor 24" decreases,the drain voltage of transistor 24" at a constant voltage greater thanV_(bias) (typically, V_(bias) is on the order of -2 Vdc). This resultsin a detector voltage of approximately zero volts if V_(bias) is chosencorrectly. The decay, due to the MOSFET characteristics shown in FIG. 6,proceeds rapidly so that over most of the switching cycle V_(det) isapproximately zero.

Since V_(det), on the average, has a reduced value, Z_(in), theequivalent input impedance to tranresistance amplifier 8" also has areduced value as compared with the input impedance of prior arttransresistance amplifiers. With R_(in) considerably less than R_(det),the injection efficiency, n_(eff), is very high. In other words,substantially all of I_(ph) flows into transresistance amplifier 8". Thegain provided by the gain stage (transistor 24") and its associatedswitched capacitor 38 thus results both in V_(det) being reduced towardszero and in an increase in n_(eff). The actual value of gain is notcritical since the primary function of the above-describedtransresistance amplifier is to provide an accurate measurement ofI_(ph).

It has been determined that the total transresistance R_(t) (FIG. 4) ofamplifier 29 is approximately 1/f_(s) C₁₂ wherein f_(s) is the switchingfrequency of signal .0.₂ (t), typically 5.0 Hz, and C₁₂ is thecapacitance value for R_(t) of 20 Gohms. Thus, R_(t) can be chip tunedto provide a desired change in V_(o) for a given change in I_(ph).

Typical component types and values for a monolithic version of theschematic circuit shown in FIG. 4 have been determined to be as follows:

    V.sub.rst =10 Vdc

    V.sub.dd =3 Vdc ##EQU2##

    C.sub.38 =2.5 pf

    C.sub.12 =10 pf

    C.sub.20 =2.5 pf

Detector--HgCdTe type manufactured by Santa Barbara Research Center,Goleta, Calif.

A scaled version of the circuit shown in FIG. 4 using discrete activecomponents, and having a polarity the reverse of the circuit shown inFIG. 4 (p-channel MOSFETs were used rather than n-channel MOSFETs), wassuccessfully tested at 77° K. The component types and values in thiscircuit were as follows:

    V.sub.rst =-3.90 Vdc

    V.sub.dd =-10.0 Vdc

    V.sub.bias =+3.38 Vdc ##EQU3##

    C.sub.38 =24 pf

    C.sub.12 =93.1 pf

    C.sub.20 =24 pf

All discrete MOSFETs were type 3N163 manufactured by SiliconixIncorporated, Santa Clara, Calif.

A 200 MΩ resistor was substituted for the detector for this test.

The present invention thus provides a transresistance amplifier which isa significant improvement over the prior art. By using switchedcapacitor resistances in the output load and in the gain stage insteadof discrete resistors, implementation in monolithic form with much lowerpower dissipation on a smaller chip area can be easily achieved.Further, the specific arrangement of components in the amplifier allowMOSFET transistor current to be controlled accurately and rapidly,decreasing from a relatively high value to a low value very rapidly.This feature is particularly useful when converting the current outputfrom a photovoltaic detector to a voltage while maintaining the averagedetector voltage close to zero, thus minimizing certain types ofdetector noise. Reducing the detector voltage to approximately zero alsosubstantially reduces the effective input impedance of the amplifierthus increasing the injection efficiency of the amplifier. Further,utilizing a switched capacitor resistor as the output load enables thetotal transresistance of the amplifier to be set to a desired value bysimply tuning the switched capacitor value until the desired value oftransistance is attained.

While the invention has been described with reference to its preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the true spirit and scope of theinvention. In addition, many modifications may be made to adapt aparticular situation or material to the teaching of the inventionwithout departing from its essential teachings.

What is claimed is:
 1. An improved transresistance amplifier providinglow input impedance comprising:a first transistor having an inputterminal, an output terminal and a gate, said input terminal thereofbeing coupled to an input port of the transresistance amplifier, saidoutput terminal thereof being coupled to an output port of thetransresistance amplifier; a first capacitor coupled to the output portof the transresistance amplifier; switched voltage means coupled to saidfirst capacitor, said switched voltage means periodically resetting avoltage across said capacitor to a first predetermined reference level;and feedback amplifier means coupled between said gate and said inputterminal of said transistor, said feedback amplifier means having aswitched capacitor load operatively coupled thereto and to said gate toprovide gain to said amplifier for reducing a voltage across the inputport of the transresistance amplifier in response to a current appliedto the input port of the transresistance amplifier; whereby the voltageacross said first capacitor may be periodically sampled before beingreset to obtain an indication of the current applied to the input portof said transresistance amplifier.
 2. The transresistance amplifier ofclaim 1, wherein said feedback amplifier means comprisea secondtransistor having an input terminal, an output terminal and a gate, saidinput terminal thereof being coupled to a first reference voltage sourcehaving a second predetermined level, said gate thereof being coupled tosaid input terminal of said first transistor, and said output terminalthereof being coupled to said gate of said first transistor and having aswitched capacitor load coupled thereto.
 3. The transresistanceamplifier of claim 2 wherein said switched capacitor load comprises:athird transistor having an input terminal, an output terminal and agate, said input terminal thereof being coupled to a second referencevoltage source having a third predetermined level, said output terminalthereof being coupled to said output terminal of said second transistor,said gate thereof being coupled to a first switching signal sourceproviding periodically recurring singals for gating said thirdtransistor alternately on and off; and a second capacitor coupledbetween said output terminal of said second transistor and the commonground of the transresistance amplifier.
 4. The transresistanceamplifier of claim 3 wherein the periods of said switching signal sourceand of said switched voltage means are the same.
 5. The transresistanceamplifier of claim 4 wherein said switched voltage means comprise afourth transistor having an input terminal, an output terminal and agate, said input terminal thereof being coupled to a third referencevoltage source having a fourth predetermined level, corresponding tosaid first predetermined level, said output terminal thereof beingcoupled to that terminal of said output port not connected to the commonground of the transresistance amplifier, and said gate being coupled tosaid switching signal source.
 6. The transresistance amplifier of claim1 wherein said first transistor comprises a MOSFET.
 7. Thetransresistance amplifier of claim 3 wherein said first, said second andsaid third transistors comprise MOSFETs.